1. Field of the Invention
The present invention relates to an exhaust gas purifying method and apparatus for purifying exhaust gas emitted from an internal combustion engine.
2. Description of the Related Art
In an automotive internal combustion engine or the like, it is required to purify the components of exhaust gas, such as carbon monoxide (CO), nitrogen oxide (NOx), and hydrocarbon (HC) before their emission to the atmosphere.
To meet such a requirement, an exhaust gas purifying apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. Hei 5-171929 is known. In this exhaust gas purifying apparatus, there is provided an exhaust system in which an exhaust pipe downstream of a catalyst is branched into a first branch flow path and a second branch flow path and these branch flow paths are again merged into a single flow on the downstream side. Then, an adsorbent for adsorbing the hydrocarbon is provided in the first branch flow path, and an exhaust gas recirculation path is provided downstream of the adsorbent for introducing a part of the exhaust gas to the intake side of the internal combustion engine.
Then, in the exhaust gas purifying apparatus, in the case where the catalyst is kept under non-active conditions as in the case where the internal combustion engine is started in a cold condition, the exhaust gas from the internal combustion engine is caused to flow from the catalyst to the first branch flow path and to a muffler through the adsorbent until the catalyst is activated. Thus, the hydrocarbon (HC) contained in the exhaust gas is adsorbed to the adsorbent.
Thereafter, when the catalyst is heated by the heat of the exhaust gas and the temperature of the catalyst reaches the activation temperature, the exhaust gas purifying apparatus renders the exhaust gas, which has been purified by the catalyst, to flow through both first branch flow path and second branch flow path so that the exhaust gas, which has been introduced in the first branch flow path, may flow through the adsorbent to the exhaust gas recirculation flow path and the exhaust gas, which has been introduced into the second branch flow path, may flow to the muffler.
In this case, the exhaust gas introduced into the first branch flow path causes the adsorbent to be heated and removes therefrom the hydrocarbon (HC) adsorbed to the adsorbent. The hydrocarbon (HC) separated from the adsorbent is introduced into the exhaust gas recirculation flow path together with the exhaust gas and recirculated to the intake side of the internal combustion engine.
By the way, not only the hydrocarbon (HC) but also adhered materials such as soot which have passed the catalyst are adhered to the adsorbent. Also, the internal combustion engine is at a standstill before the separation of the hydrocarbon (HC) from the adsorbent has been completed, the hydrocarbon (HC) is kept adsorbed to the adsorbent. Then, in the next start of the internal combustion engine, if an engine RPM is increased immediately after the start, since the high temperature exhaust gas passes through the adsorbent, a part of the hydrocarbon (HC) adsorbed to the adsorbent becomes soot and again adheres to the adsorbent.
The adhered material thus adhered to the adsorbent causes that the adsorbing performance is degrated. It is therefore necessary to remove the adhered material away from the adsorbent. However, in the above-described exhaust gas purifying apparatus, since the adsorbent is heated by the low flow rate such as the exhaust gas recirculation, it is difficult to heat the adsorbent up to a temperature range in which the adhered materials such as soot may be removed from the adsorbent.